The present invention relates generally to video processing systems. More specifically, the present invention includes a method and apparatus for processing a video sequence that automatically reconstructs the original frames from the output of a preceding telecine process.
The film used to record and project motion pictures is divided into a sequence of individual images known as frames. Video tape is encoded using a similar sequence of frames. As a result, it might appear that converting between film and video tape would be a simple process of transferring frames from the source media to equivalent frames within the destination media. Unfortunately, film and video tape are recorded using different frame rates. For film, the rate is typically twenty-four frames for each second. Video tape encoded using the NTSC standard uses a rate of approximately thirty frames per second. Video tape encoded using the PAL standard uses a rate of approximately twenty-five frames per second.
The different frame rates used by film and video complicates the conversion between the two types of media. Specifically, when converting from film to video tape, each second of film must be expanded from twenty-four to thirty frames. Effectively, an extra, or composite video frame must be made up or fabricated for each four frames of film. When converting from video tape to film, each second of video tape must be reduced from thirty to twenty-four frames. This means that one frame out of every five frames of video tape must be discarded.
The process of converting from film to video is known as telecine. Typical telecine processes create composite frames using an interlacing method. The interlacing method takes advantage of the fact that video frames are divided into an odd field and an even field. The even field includes all of the even lines within the video frame. The odd field includes all of the odd lines within the frame. An extra, or composite frame is created by combining the odd field from one frame with the even field from an adjacent second frame. The composite frame is is intended to approximate a midway point between frames and is inserted between the first and second frames.
The frames added by the telecine process make recordings made on film viewable in video formats. There are many cases, however, where it is desirable to remove the frames added during the telecine process. One of these cases At arises when video data needs to be compressed. Compression is used where it is necessary to reduce the number of bits used to digitally encode video data. Compression increases the capacity of storage devices, such as optical disks. Compression also increases the performance of video transmissions, such as satellite or Internet transmissions. Unfortunately, the composite frames produced by the telecine process tend to disrupt the compression process. This occurs because the output frame rate is higher (resulting in a greater amount of data), and because composite frames tend to be different than both their preceding and following frames. These differences introduce additional discontinuities in the sequence of frames. Video compression methods, such as MPEG, are most effective when applied to relatively continuous frame sequences. These compression methods exhibit lower efficiencies when they are applied to sequences of frames that have been processed through the telecine process.
The traditional method for removing composite frames requires an operator to manually detect the first composite frame by searching through the sequence of frames encoded on a video tape. The composite frame is then discarded. Each following fifth frame may then be discarded as well. In practice, this method suffers from at least two deficiencies. The first is the requirement that an operator manually scan through frames looking for the initial composite frame. The second, and potentially more serious, deficiency arises when composite frames are to be removed from a video tape that has been edited after the telecine process. In these cases, frames within the video tape may have been removed or added. This means that composite frames may occur at rather random intervals and not every fifth frame. The result is that the process of manually selecting the initial composite frame may have to be repeated. If a video tape includes a large number of edits, the manual selection process may become extremely time consuming or impractical.
As a result, it is clear that there is a need for a process that automatically removes the composite frames generated by the telecine process. This is especially important where the composite frames cannot be assumed to occur on a strictly periodic basis, as the case of an edited video tape.
An embodiment of the present invention provides a method and apparatus for performing an inverse telecine process. The inverse telecine process automatically detects and replaces composite frames created during a preceding telecine process. For the method of the present invention, a computer system reads each of the frames in a source video sequence. The computer system uses the frames to create an output video sequence.
During this process, the computer system analyzes the pixels included in each frame read from the source video sequence. If the pixels included in the even field of a frame correspond to the adjacent pixels included in the odd field, the computer system concludes that the frame is original. An original frame includes pixels from only one image. Original frames are added, without modification, to the destination video sequence.
If the pixels included in the even field of a frame do not correspond to their adjacent pixels included in the odd field, the computer system concludes that the frame is composite. A composite frame includes pixels from two distinct images and is created during a telecine process. When a composite frame is detected, the computer system attempts to reconstruct the composite frame to form an original frame.
To reconstruct composite frames into original frames, the computer system compares the pixels included in the composite frame to the pixels included in the frame that precedes the composite frame in the source video sequence. If the pixels included in the even field of the composite frame correspond to their adjacent pixels included in the odd field of the preceding frame, the computer system reconstructs an original frame using the even field of the composite frame and the odd field of the preceding frame. If the pixels included in the odd field of the composite frame correspond to their adjacent pixels included in the even field of the preceding frame, the computer system reconstructs an original frame using the odd field of the composite frame and the even field of the preceding frame. The computer system adds reconstructed frames to the destination video sequence.
The computer system creates interpolated frames for composite frames that cannot be reconstructed. The computer system creates interpolated frames by applying an interpolation function to the pixels included in the composite frames. The computer system adds interpolated frame to the destination video sequence only if they cannot be discarded without compromising the synchronization between the source and destination video sequences.
The method of the present invention performs the inverse telecine process using only a single pass and is self-adapting to a variety of different input and output frame rates. The automatic detection of composite frames means that the method is self-synchronizing and automatically adapts to discontinuities introduced during the editing process.
Advantages of the invention will be set forth, in part, in the description that follows and, in part, will be understood by those skilled in the art from the description herein. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims and equivalents.